English

Noun

uncle-in-law ‎(plural uncles-in-law)

1. (nonstandard) The husband of one's aunt.
   • 1871, William Lindsay, Inquiry into the Christian law, as to the relationships which bar marriage p. 86

     Between uncle and aunt-in-law, or uncle's wife, you have a connection of affinity, and you have the same between aunt and uncle-in-law, or aunt's wife. [sic]

2. (nonstandard) The uncle of one's spouse.
   • 2012 August 11, anonymous, Vernon Dursley, Harry Potter Wiki

     Vernon would later become an uncle-in-law to Harry's wife, Ginny and a great-uncle to Harry and Ginny's children, James Sirius Potter, Albus Severus Potter and Lily Luna Potter.

Usage notes

Both meanings commonly used for uncle-in-law are used to define a distinction between someone who is an uncle by marriage (i.e., an in-law) and someone who is an uncle by blood (i.e., a brother of one of your parents). However both meanings are included within the standard definition of uncle, and for those who use this term, and a user may be trying to convey only one or both of the meanings given here. In general, unless there is a need for specificity, the use of uncle is to be preferred.